Accumulator fuel-injection apparatus

ABSTRACT

A controller ( 8 ) of an accumulator fuel-injection apparatus opens a selector valve ( 5 ) for injection rate switching for a short time between auxiliary injection and main injection, the selector valve controlling high-pressure fuel supply from a high-pressure accumulator ( 3 ) to a fuel passage ( 10   a ) on the downstream side of a check valve ( 32 ), whereby a fuel pressure higher than an auxiliary injection pressure and lower than a main injection pressure is formed in the fuel passage on the downstream side of the selector valve ( 5 ). By opening, in this state, an on-off valve ( 7 ) for injection timing control to start the main injection, a proper pressure higher than the auxiliary injection pressure and lower than the subsequent main injection pressure is established in the initial stage of main injection, improving fuel economy and exhaust-gas characteristic.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/JP00/02108 which has an Internationalfiling date of Mar. 31, 2000, which designated the United States ofAmerica.

TECHNICAL FIELD

The present invention relates to an accumulator fuel-injectionapparatus.

BACKGROUND ART

An accumulator fuel-injection apparatus is known which stably suppliesrespective cylinders of a diesel engine with high-pressure fuel storedin an accumulator to improve engine performance in a broad operatingregion. Even with a fuel-injection apparatus of this type, an abruptexplosion combustion takes place in the initial stage of combustion,causing noisy engine operation and increased NOx in exhaust gas, if thefuel injection rate immediately after the start of fuel injection is toohigh or if the quantity of fuel injected in an ignition delay period istoo large.

As a countermeasure against such a problem, auxiliary injection can beperformed prior to main injection in each fuel injection cycle. Thisshortens the ignition delay period, realizing a reduction in an amountof fuel injected in the ignition delay period by the main injection,whereby the abrupt combustion is prevented to reduce the engine noiseand the NOx emission. In case that the fuel-injection apparatus isconfigured to perform high-pressure injection from the beginning of themain injection following the auxiliary injection, however a satisfactoryreduction in noise and NOx emission cannot be always achieved. To attainthe ignition-delay reducing effect, a minimum required quantity of fuelshould be injected in the auxiliary injection. In order to reduce thefuel injection quantity to the required minimum in the fuel-injectionapparatus of a type performing the auxiliary injection at a relativelyhigh injection pressure, the auxiliary injection period must beshortened. This requires high control accuracy. If the required controlaccuracy is not attained, the fuel injection quantity in the auxiliaryinjection becomes too small or too large, so that the intended effectsof the auxiliary injection may not be achieved, resulting in degradedexhaust gas and fuel economy.

As another means of reducing engine noise and NOx emission, anaccumulator fuel-injection apparatus has been proposed that injects fuelat a lower fuel injection rate in the initial stage of fuel injection ineach fuel injection cycle. Byway of example, the proposed apparatus hasa low-pressure accumulator for storing low-pressure fuel, ahigh-pressure accumulator for storing high-pressure fuel, a selectorvalve for selectively communicating the low- or high-pressureaccumulator with an injector (fuel injection nozzle) to switch injectionrate, and an on-off valve for permitting/preventing communicationbetween a control chamber of the injector and a fuel tank to controlinjection start/end timings.

An accumulator fuel-injection apparatus of this kind, such as forexample an apparatus disclosed in International Publication No.WO98/09068, is designed to control opening/closing timings of an on-offvalve for injection timing control and of a selector valve for injectionrate switching, so as to carry out only main injection or both maininjection and auxiliary injection in each fuel-injection cycle. Inaddition, there is disclosed a technique of performing low-pressureinjection in the initial stage of the main injection followed byhigh-pressure injection.

In connection with the present invention, the apparatus disclosed in theabove publication can perform low-pressure auxiliary injection for ashort time, and start main injection when a predetermined period of timehas elapsed after completion of the auxiliary injection. Low-pressureinjection is carried out in the initial stage of the main injection, andthen high-pressure injection is carried out over the remaining period.

Specifically, by closing the on-off valve for injection timing controland the selector valve for injection rate switching, a fuel passageconnecting the selector valve and a fuel chamber of the injector isfilled with low-pressure fuel and a control chamber of the injectorcommunicated with the fuel passage is supplied with the low-pressurefuel, whereby the injector is maintained in a valve-closing state. Whenthe auxiliary-injection start timing is reached, the on-off valve isopened to discharge the fuel in the control chamber to the fuel tank,whereby the injector is opened to make low-pressure auxiliary injection,and the on-off valve is closed when the auxiliary injection period haselapsed. When a predetermined period of time has elapsed aftercompletion of the auxiliary injection so that the main-injection starttiming is reached, the on-off valve is opened again to startlow-pressure main injection, and the selector valve is opened in thecourse of main injection so that high-pressure fuel supplied from thehigh-pressure accumulator is injected through the nozzle, therebyeffecting high-pressure injection.

By conducting the low-pressure auxiliary injection and the maininjection including low- and high-pressure injections in each fuelinjection cycle as described above, the fuel economy and exhaust-gascharacteristic of the engine can be improved. However, there is a demandfor further improvement in fuel economy and exhaust-gas characteristic.

DISCLOSURE OF THE INVENTION

The object of the present invention is to provide an accumulatorfuel-injection apparatus capable of establishing a proper injectionpressure in the initial stage of main injection following low-pressureauxiliary injection, to thereby further improve fuel economy andexhaust-gas characteristic.

In order to attain the above object, an accumulator fuel-injectionapparatus according to the present invention comprises a firstaccumulator for storing high-pressure fuel; a control valve forcontrolling discharge of the high-pressure fuel stored in the firstaccumulator toward a downstream-side of a fuel passage; a secondaccumulator, communicated with the fuel passage on the downstream of thecontrol valve, for storing low-pressure fuel; and fuel control means foropening the control valve for a short time between auxiliary shortinjection and main injection, for opening the control valve in thecourse of the main injection, and then closing the control valve inconformity with completion of the main injection, while the auxiliaryshort injection and the main injection are carried out in this orderwith an interval through a fuel injection nozzle communicated with thefuel passage.

Desirably, the fuel control means establishes an intermediate pressurein the fuel passage on the downstream-side of the control valve in theinitial stage of the main injection, the intermediate pressure beinghigher than the pressure of the low-pressure fuel in the secondaccumulator and lower than the pressure of the high-pressure fuel in thefirst accumulator.

With the accumulator fuel-injection apparatus of the present invention,when the control valve is opened for a short time after completion ofthe low-pressure auxiliary injection, the fuel passage is supplied withhigh-pressure fuel from the first accumulator for a short time, so thatthe fuel pressure in the fuel passage becomes higher than anauxiliary-injection pressure. In the initial stage of the main injectioneffected through the fuel injection nozzle, therefore, fuel is injectedat a pressure higher than the auxiliary-injection pressure, desirably atan intermediate pressure higher than the auxiliary-injection pressureand lower than the pressure of the high-pressure fuel. When the controlvalve is opened in the course of the main injection, the high-pressurefuel is supplied through the fuel passage to the fuel injection nozzle,so that high-pressure injection is performed.

By injecting fuel in the initial stage of the main injection followingthe low-pressure auxiliary injection, at a pressure higher than theauxiliary-injection pressure, desirably at an intermediate pressurehigher than the auxiliary-injection pressure and lower than the pressureof the high-pressure fuel, the fuel injection quantity in the initialstage of the main injection increases as compared with the case wherelow-pressure injection is performed in the initial stage of the maininjection. Consequently, the fuel quantity to be injected in theremaining period of the main injection decreases by the quantitycorresponding to the increase in the injection quantity in the initialstage, and hence the period of the entire main injection is shortened.By injecting the appropriate quantity of fuel in the initial stage ofthe main injection to shorten the main injection period in this way, thefuel injection ends early and therefore the fuel economy is improved. Inaddition, unlike the case where high-pressure injection is started fromthe initial stage of the main injection, excessive fuel supply beforeignition is prevented, resulting in reduction in engine noise and NOxemission.

Furthermore, by conducting the auxiliary injection at a low pressure,the required accuracy of auxiliary-injection time control is alleviatedas compared to the case where the auxiliary injection is performed at ahigh pressure, so that the fuel injection quantity in the auxiliaryinjection may be more accurately controlled to the required minimum,contributing to improvement in fuel economy.

The fuel control means may be so configured, for instance, as to controlopening/closing timings of valves for fuel injection timing control andfor injection rate switching. This eliminates the need of utilizing aparticularly complicated arrangement.

FIGS. 1 to 3 schematically show the states of fuel spray formed by theauxiliary injection, the initial stage of the main injection, and thesubsequent main injection, respectively, based on results of acombustion observation experiment the present inventors conducted. Inthe experiment, observations were made from above a cylinder on fuelspray formed by the fuel injected from a fuel injection nozzle with fournozzle holes. In the illustrations, a small circle represents the fuelinjection nozzle, and a large half circle represents half of thecylinder.

In the present invention, low-pressure auxiliary injection is performedprior to main injection, as described above. The auxiliary injection ismade in a condition that a piston is on the lower side in the cylinderso that the density of gas in the cylinder is low. This allows fuelspray to easily diffuse outward in the radial direction of the cylinder,however, the diffusion of the fuel spray is restrained appropriatelysince the auxiliary-injection pressure is low. As shown in FIG. 1, thefuel spray is distributed in an area from the vicinity of the fuelinjection nozzle to a radially intermediate part of the cylinder.

As described above, some conventional accumulator fuel-injectionapparatuses are designed to inject fuel in the initial stage of maininjection at a low pressure equivalent to auxiliary-injection pressure.With this arrangement, since the area to which fuel spray can reach(i.e., fuel spray distribution) overlaps the fuel spray distributioncaused by the auxiliary injection, too much fuel may exist in that area.This causes a fear that the area is not supplied with sufficient airrequired for simultaneous or successive combustion of the fuel spraysformed by the auxiliary injection and the low-pressure main injection.In particular, the air for combustion of the fuel spray associated withthe low-pressure main injection is consumed during the combustion of thefuel spray produced by the auxiliary injection, causing shortage of air,if the fuel spray produced by the auxiliary injection is caused toignite, just before or after the start of the main injection, withthe,increase in pressure and temperature in the cylinder due to upwardpiston movement. At any rate, when fuel is injected at a low pressure inthe initial stage of the main injection, the fuel spray and flamescaused by the auxiliary injection hinder the diffusion of the fuel sprayformed by the low-pressure main injection and hinder the supply of freshair required for combustion of the fuel spray, preventing a propercombustion of the fuel spray. Thus, black smoke is liable to bedischarged from the engine.

In contrast, according to the present invention, the main injection isstarted in a condition that a fuel pressure higher than theauxiliary-injection pressure is established in the fuel passage on thedownstream-side of the control valve, as described above. Since the fuelinjection pressure in the initial stage of the main injection is higherthan the auxiliary-injection pressure, the fuel spray reaches, as shownin FIG. 2, an area outwardly of the fuel spray and flames formed by theauxiliary injection as viewed in the radial direction of the cylinder.Sufficient air remaining in the outer area permits a proper combustionof the fuel spray. While the fuel injected at an intermediate pressureis scattered to penetrate through the fuel spray and flames formed bythe auxiliary injection as described above, surrounding air is involvedin the fuel spray, so that the volume of the entire fuel sprayincreases. This means that the fuel spray diffuses satisfactorily in thecylinder to provide an appropriate fuel spray distribution in thecylinder.

Under such a condition, the fuel injection pressure is changed fromintermediate pressure to high pressure. The high-pressure fuel sprayproperly diffuses in the cylinder as indicated in black in FIG. 3, whileincreasing, its volume by sucking surrounding air and combustionresidues such as soot produced by the auxiliary injection and theintermediate-pressure main injection. Since the diffusing high-pressurefuel spray activates the combustion in the cylinder, the entire fuelspray burns satisfactorily, preventing the production of black smoke.Referring to FIG. 3, the distribution area of the high-pressure fuelspray is dislocated from that of the intermediate pressure fuel spay inthe circumferential direction of the cylinder. Presumably, this iscaused by a swirl formed in the cylinder.

As described above, according to the present invention, theintermediate-pressure main injection and the high-pressure maininjection are performed successively in a condition that the diffusionof fuel spray formed in the cylinder by low-pressure auxiliary injectionis restrained appropriately, so as to realize a proper combustion of thefuel spray. This makes it possible to improve the fuel economyand-exhaust-gas characteristic of the engine and reduce the enginenoise.

In the present invention, desirably, the intermediate pressure formed inthe fuel passage on the downstream-side of the control valve in theinitial stage of the main injection by the fuel control means has avalue closer to the pressure of the low-pressure fuel in the secondaccumulator in a lower-speed, lower-load engine operating region, andcloser to the pressure of the high-pressure fuel in the firstaccumulator in a higher-speed, higher-load engine operating region.

Generally, the distance for which the injected fuel flies is longer in alower-speed, lower-load operating region where the in-cylinder pressureis low, whereas the injected fuel flies over a smaller distance in ahigher-speed, higher-load operating region where the in-cylinderpressure is high. In the preferred arrangement, the injection pressureof the intermediate-pressure main injection is varied depending on theengine operating state. Thus, the pressure of injected fuel, thedistance for which the fuel flies, and the diffusion state of fuel sprayin the initial stage of the main injection are suited to the engineoperating state.

Desirably, the fuel control means controls the pressure of thehigh-pressure fuel in the first accumulator and the pressure of thelow-pressure fuel in the second accumulator, based on the operatingstate of the engine, for example, the speed and load of the engine. Inthis case, the intermediate pressure formed in the fuel passage on thedownstream-side of the control valve in the initial stage of the maininjection is suited to the engine operating state.

Desirably, the fuel control means variably controls the period of timefor which the control valve is opened between the auxiliary injectionand the main injection. The control-valve opening time period determinesthe intermediate pressure formed in the fuel passage on thedownstream-side of the control valve in the initial stage of the maininjection. Thus, the intermediate pressure can be varied accurately andeasily by the variable control of the control-valve opening period.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing, in respect of half of acylinder, the state of fuel spray at the time of auxiliary injection;

FIG. 2 is a schematic illustration showing the state of fuel spray atthe time of intermediate-pressure main injection;

FIG. 3 is a schematic illustration showing the state of fuel spray atthe time of high-pressure main injection;

FIG. 4 is a diagram showing an accumulator fuel-injection apparatusaccording to an embodiment of the present invention;

FIG. 5 is a diagram showing connection between main elements of thefuel-injection apparatus shown in FIG. 4 and respective injectors ofcylinders of an engine;

FIG. 6 is a diagram showing injection waveform for one fuel-injectioncycle performed by the fuel injection apparatus shown in FIGS. 4 and 5,along with changes with elapse of time in on/off states of an injectordrive signal and a selector-valve drive signal and in injector inletpressure;

FIG. 7 is a flowchart showing an injector/selector-valve control routineexecuted by a controller shown in FIGS. 4 and 5;

FIG. 8 is a diagram showing that the main-injection time decreasingeffect attained by conducting the intermediate-pressure main injectionand high-pressure main injection following low-pressure auxiliaryinjection:

FIG. 9 is a diagram showing maps for low-pressure auxiliary injection,intermediate-pressure main injection and high-pressure main injectionfor use in determining required pressures that varies depending onengine speed and engine load; and

FIG. 10 is a graph showing a relationship between required pressure inintermediate-pressure main injection and high-pressure fuel feed periodAT for establishing intermediate pressure.

BEST MODE OF CARRYING OUT THE INVENTION

An accumulator fuel-injection apparatus according to an embodiment ofthe present invention will be described below.

The accumulator fuel-injection apparatus is incorporated in, forexample, an in-line six-cylinder diesel engine (not shown), and includesa high-pressure pump 1 as shown in FIGS. 4 and 5. The high-pressure pump1 is driven by the engine to draw fuel from a fuel tank 17 andpressurize it. The high-pressure pump 1 is, for example, apositive-displacement plunger pump whose fuel discharge pressure can beregulated by regulating an effective length of force-feed stroke. Theforce-feed stroke is regulated, for example, by adjusting the timing ofclosing a solenoid valve (not shown). While the solenoid valve is open,!pressure-feed actions are rendered ineffective. The high-pressure pump 1of the apparatus of the present embodiment adapted for a six-cylinderengine has, for example, two plungers. Each plunger is associated withthree cylinders and makes three force-feed strokes during one rotationof a high-pressure pump shaft.

A controller (ECU) 8 of the accumulator fuel-injection apparatusvariably regulates the force-feed stroke of the pump 1 based on enginespeed Ne and accelerator-pedal depression (an accelerator openingdegree) ACC detected by an engine speed sensor 8 a and an acceleratoropening-degree sensor 8 b, respectively, and feedback-controls theforce-feed stroke (fuel pressure) based on actual pressure P_(HP) in ahigh-pressure accumulator (first accumulator) 3 detected by a pressuresensor 3 a (FIG. 2), to thereby obtain a high-pressure fuel suited toengine operating state.

The fuel pressurized by the pump 1 is stored in the high-pressureaccumulator 3. The high-pressure accumulator 3 is provided in common forall the cylinders and connected with fuel passages 10 a. The fuelpassages 10 a are each provided with a selector valve (control valves)5, such as for example two-way solenoid valve, for switching the fuelinjection rate. A check valve 32 is also provided in the fuel passage 10a on the side immediately downstream of the selector valve 5.

The fuel passage 10 a is connected with a low-pressure accumulator(second accumulator) 4, provided for all the cylinders, through a fuelpassage 10 b branched from the fuel passage 10 a on the downstream sideof the check valve 32. A check valve 6 arranged in the fuel passage 10 bis designed to open when the fuel pressure in the fuel passage 10 b onthe side close to the low-pressure accumulator 4 is higher than the fuelpressure in the fuel passage 10 a on the downstream side of the checkvalve 32. The fuel passage 10 b is formed with a bypass fuel passagebypassing the check valve 6 and having an orifice 6 a. When the fuelpressure in the fuel passage 10 a is higher than the fuel pressure inthe fuel passage 10 b, the fuel in the fuel passage 10 a flows into thefuel passage 10 b through the orifice 6 a, and then flows into thelow-pressure accumulator 4. Between the low-pressure accumulator 4 andthe fuel tank 17 is arranged a pressure control solenoid valve 34 (FIG.4) for controlling the fuel pressure in the low-pressure accumulator 4to a predetermined pressure.

In place of the pressure control valve 34 shown in FIG. 4, a pressurecontrol valve 34 (FIG. 5) may be used which regulates, under the controlof the controller 8, the fuel pressure in the low-pressure accumulator 4to a predetermined pressure. In the following, the fuel injectionapparatus with the pressure control valve 34 shown in FIG. 5 will bedescribed. In FIG. 5, reference numeral 4 a denotes a pressure sensorfor detecting a fuel pressure P_(LP) in the low-pressure accumulator 4.

The controller 8 controls the pressure control valve 34 based on anactual fuel pressure P_(LP) detected by the fuel sensor 4 a (FIG. 5) soas to establish the fuel pressure in the low-pressure accumulator 4suited to engine operating state represented by engine speed Ne andaccelerator pedal depression ACC.

Each of injectors (fuel injection nozzles) 9 provided for every cylinderof the engine has a control chamber 11 and a fuel chamber 12 that areconnected with the fuel passage 10 a. The control chamber 11 isconnected with the fuel tank 17 through a fuel return passage 10 c.Reference numerals 15, 16 denote orifices. Reference numeral 7 denotesan on-off valve arranged in the fuel return passage 10 c, such as forexample a two-way solenoid valve, for controlling the fuel injectiontiming. The on-off valve 7 may be incorporated in the injector.

The injector 9 has a needle valve 13 that is movable, under the fuelpressure supplied to the fuel chamber 12, in the direction to open anozzle hole, a hydraulic piston 14 movable in the direction to close thenozzle hole under the fuel pressure supplied to the control chamber 11,and a spring (not shown) urging the needle valve in the direction toclose the nozzle hole.

When the control chamber 11 and the fuel chamber 12 are supplied withfuel of the same pressure from the fuel passage 10 a and the on-offvalve 7 for fuel-injection timing control is closed, the sum of theforces produced by the fuel pressure and the spring and acting on thehydraulic piston 14 is larger than the force produced by the fuelpressure and acting on the needle valve 13, and hence the needle valve13 closes the nozzle hole. On the other hand, when the on-off valve 7 isopen so that the fuel in the control chamber 11 is discharged to thetank 17, the force acting on the hydraulic piston 14 reduces ordisappears, and hence the needle valve 13 pushes the hydraulic piston 14upward to open the nozzle hole, so that the fuel in the fuel chamber 12is injected into a combustion chamber (not shown) of the engine.

The accumulator fuel-injection apparatus of the present embodiment,configured to carry out main injection and auxiliary injection precedingthe main fuel injection, mainly contemplates shortening the maininjection time period without excessively increasing the injectionpressure in the initial stage of main injection, particularly theinjection pressure before ignition, and also contemplates supplying, bythe auxiliary injection, the minimum quantity of fuel required toprevent an ignition delay of fuel supplied by the main injection,thereby simultaneously achieving improved fuel economy, reduced enginenoise, and reduced NOx emission.

Specifically, as shown in the uppermost part of FIG. 6, in each fuelinjection cycle, the fuel injection apparatus starts the main injectionat an intermediate pressure when a predetermined period of time haselapsed after completion of auxiliary injection at a low pressure, andswitches the injection pressure from the intermediate pressure to highpressure during the main injection.

As for the fuel injection control, the controller 8 determines an engineoperating state based on, for example, engine speed Ne and acceleratorpedal depression Acc. Based on the determined engine operating state,the controller 8 controls the timing of opening/closing a solenoid valve(not shown) for force-feed stroke adjustment, so as to regulate theforce-feed stroke of high-pressure pump 1, thereby establishing a fuelpressure in the high-pressure accumulator 3 suited to engine operatingstate. The controller 8 also controls the operation of the pressurecontrol valve 34 based on the engine operating state, to thereby controlthe fuel pressure in the low-pressure accumulator 4 so as to be suitedto the engine operating state. Furthermore, the controller 8 determinesthe values of injection control parameters based on the engine operatingstate, for example, by referring to maps. As shown in FIG. 6, theinjection control parameters include, for example, auxiliary injectionstart timing t1, auxiliary injection period ΔTL (=t2−t1), high-pressurefuel feed start timing t3 to form the intermediate pressure,high-pressure fuel feed period ΔTm (=t4−t3), main injection start timingt5, intermediate pressure injection period ΔTM (=t6−t5), and maininjection period ΔTMH (=t7−t6). Based on the injection control parametervalues, the controller 8 further determines injector drive signalturning-on timings t1, t5, injector drive signal turning-off timings t2,t7, selector-valve drive signal turning-on timings t3, t6, andselector-valve drive signal turning-off timings t4, t8.

Next, the operation of the fuel injection apparatus having theabove-described structure will be described.

During the engine operation, the controller 8 executes fuel pressurecontrol routine (not shown) and injector/selector-valve control routineshown in FIG. 7 in parallel and periodically. Both the control routinesstart each time the time point to start the fuel injection cycle foreach cylinder is reached, which point is recognized, for example, basedon the elapsed time after a cylinder discrimination signal or a crankshaft rotational position signal (not shown) rise.

For convenience of illustration and explanation, the control routineshown in FIG. 7 includes only a control procedure for one cylinder. Partof the control procedure, for example, the determination of engineoperating state may be performed for all the cylinders only once in onefuel-injection cycle.

In the fuel pressure control routine, the solenoid valve for force-feedstroke adjustment and the pressure control valve are controlled in theiropening/closing timing and valve opening degree, based on the determinedengine operating state, so that respective fuel pressures in theaccumulators 3, 4 may become their respective target pressures suited toengine operating state. In the fuel pressure control routine of thepresent embodiment, the ECU 8 reads, as engine operating-stateparameters, an engine speed Ne detected by the engine speed sensor 8 aand an accelerator opening degree ACC detected by the acceleratoropening-degree sensor 8 b and representing engine load, and determinestarget high- and low-pressure fuel pressures corresponding to thedetected engine speed Ne and accelerator opening degree ACC, byreferring to HP and LP maps shown in FIG. 9, respectively.

As shown in FIG. 9, the target low-pressure fuel pressure in the LP mapincreases with the increase in engine speed and with the increase inengine load. The target high-pressure fuel pressure in the HP map ishigher than the target low-pressure fuel pressure in the LP map at thesame engine speed and engine load.

The target high-pressure fuel pressure increases with the increase inengine speed and engine load at higher increase rates than the rate atwhich the target low-pressure fuel pressure increases as the enginespeed and engine load increase. Generally, the LP and HP maps are in theform of curved surfaces in a three-dimensional coordinate system asshown in FIG. 9.

In general, the appropriate fuel injection quantities in the auxiliaryinjection and the main injection vary depending not only on the mainengine-operating-state parameters such as engine speed and engine load(accelerator opening degree) but also on other engine-operating-stateparameters such as engine cooling water temperature, fuel temperature,intake air temperature and EGR quantity. Desirably, the LP and HP mapsused to determine the target fuel pressure affecting the fuel injectionquantity are determined taking at least some of the above-mentionedother engine-operating-state parameters into consideration. For example,it is relatively easy to prepare the LP and HP maps considering the EGRquantity (EGR-quantity corrected maps), because the EGR quantity variesdepending on engine operating region (engine speed and load). As for thetemperature parameters, the LP and HP maps for every temperature regionmay be prepared, for instance. Alternatively, EGR-quantity correctionand various temperature corrections may be made to the target fuelpressures determined from the LP and HP maps.

After determining the target fuel pressures, the ECU 8 regulates thetiming of closing the solenoid valve for force-feed stroke adjustment,so that the actual pressure P_(HP) in the high-pressure accumulator 3detected by the pressure sensor 3 a may coincide with the targethigh-pressure fuel pressure, and regulates the timing of opening/closingthe pressure control valve 34 and the valve opening degree thereof, sothat the actual pressure P_(LP) in the low-pressure accumulator 4detected by the pressure sensor 4 a may coincide with the targetlow-pressure fuel pressure.

The injector/selector-valve control routine shown in FIG. 7 is startedeach time the time point to start fuel injection cycle is reached. Atthis time, a timer (not shown) provided in, e.g., the controller 8 isstarted to measure the time elapsed from the start of fuel injectioncycle.

In the control routine in FIG. 7, an engine speed Ne and acceleratorpedal depression ACC are read to determine an engine operating state(step S1). Then, respective values of the auxiliary injection starttiming t1, auxiliary injection period ΔTL, high-pressure fuel feed starttiming t3, high-pressure fuel feed period ΔTm, main injection starttiming t5, intermediate pressure injection period ΔTM, and maininjection period ΔTMH are determined based on the engine operating statedetermined in step S1, by referring to maps. Based on these fuelinjection control parameter values, the injector drivesignal/selector-valve drive signal turning-on/off timings t1 to t8 aredetermined (step S2).

In the present embodiment, in order to determine the intermediatepressure forming high-pressure fuel feed period ΔTm, the ECU 8determines a target main-injection initial pressure (required fuelpressure) suited to engine speed Ne and accelerator opening degree ACC,by referring to an MP map shown in FIG. 9. Next, the ECU 8 determines ahigh-pressure fuel feed period ΔTm corresponding to the required fuelpressure, from a ΔTm map shown in FIG. 10.

As shown in FIG. 9, the target main-injection initial pressure (targetintermediate-pressure fuel pressure) in the MP map is higher than thetarget low-pressure fuel pressure in the LP map and lower than thetarget high-pressure fuel pressure in the HP map, at the same enginespeed and the same engine load. The target main-injection initialpressure increases with the increase in engine speed and engine load athigher increase rates than the rates at which the target low- andhigh-pressure fuel pressures increase with the increase in engine speedand engine load. In the MP map in FIG. 9, the target main-injectioninitial pressure has a value closer to the target low-pressure fuelpressure in a lower-speed, lower-load engine operating region, andcloser to the target high-pressure fuel pressure in a higher-speed,higher-load engine operating region. This way of target pressure valuesetting is in conformity with the fact that in a lower-speed lower-loadregion, the boost pressure and the in-cylinder gas density are lower andthe distance for which the injected fuel flies is larger, whereas in ahigher-speed higher-load region, the boost pressure and the in-cylindergas density are higher and the injected fuel flying distance is smaller.As a consequence, a proper fuel spray distribution is formed in thecylinder, contributing to proper combustion. Generally, the MP map is inthe form of a curved surface in a three-dimensional coordinate systemshown in FIG. 9. As in the case of the LP and HP maps, the MP map isprepared taking the influences of EGR quantity and various temperatureparameters into consideration, or alternatively, various corrections aremade to the target pressure obtained from the MP map.

In FIG. 10, symbols HP and LP denote target high- and low-pressure fuelpressures observed when the high-pressure fuel feed period ΔTm isdetermined, which pressures vary depending on engine speed andaccelerator opening degree at that time. The relation betweenhigh-pressure fuel feed period ΔTm and intermediate pressure formed byfeeding the high-pressure fuel over that period is determined byspecifications, such as the volume of pipes, including the fuel passage10 a, in the accumulator fuel-injection apparatus and the flow rate ofhigh-pressure fuel in a state that the selector valve 5 is open. Inother words, the high-pressure fuel feed period ΔTm for obtaining therequired intermediate pressure can be obtained experimentally.

Referring to FIG. 7 again, after the determination of the injector drivesignal/selector-valve drive signal turning-on/off timings t1 to t8 instep S2 of the control routine, a determination is made as to whether ornot the auxiliary injection start timing t1 is reached based on the timeelapsed from the start of fuel injection cycle. As shown in FIG. 6, boththe selector valve 5 and the on-off valve 7 are closed until theauxiliary injection start timing t1 is reached, so that low-pressurefuel is supplied from the low-pressure accumulator 4 to the fuel passage10 a on the downstream side of the selector valve 5, to be supplied tothe control chamber 11 and the fuel chamber 12. Since the on-off valve 7is closed, the sum of the force by the fuel pressure in the controlchamber 11 acting on the hydraulic piston 14 and the force by the springacting on the needle valve is larger than the force by the fuel pressurein the fuel chamber 12 acting on the needle valve 13. Therefore, thenozzle hole of the injector 9 is closed with the needle valve 13.

Here, the fuel pressure in the fuel chamber 12 (injector inlet pressure)is kept at a pressure that is substantially the same as the pressure ofthe low-pressure fuel in the low-pressure accumulator 4. Specifically,when the fuel pressure in the fuel passage 10 a on the side downstreamof the check valve 32 becomes lower than the pressure of thelow-pressure fuel, the check valve 6 in the fuel passage 10 b opens, sothat the low-pressure fuel is supplied from the low-pressure accumulator4 to the fuel passage 10 a. On the other hand, when the fuel pressure inthe fuel passage 10 a on the downstream side of the check valve 32becomes higher than the pressure of the low-pressure fuel, the fuel inthe fuel passage 10 a flows through the orifice 6 a into thelow-pressure accumulator 4.

When the auxiliary injection start timing t1 is reached, the injectordrive signal is turned on and only the on-off valve 7 is opened (stepS3). Thus, the low-pressure fuel in the control chamber 11 is drainedthrough the orifice 16 to the fuel return passage 10 c. At the time whenthe sum of the force acting on the hydraulic piston 14 and the force bythe spring acting on the needle valve becomes smaller than the forceacting on the needle valve 13, the needle valve 13 moves up to open thenozzle hole, so that the low-pressure fuel is injected from the injector9. Thus, the auxiliary injection starts at a relatively low injectionpressure, or at a relatively low fuel injection rate (fuel injectionquantity per unit time).

Thereafter, when the auxiliary injection period ΔTL has elapsed from theauxiliary injection start timing t1 so that the auxiliary injection endtiming t2 is reached, the injector drive signal is turned off and theon-off valve 7 is closed (step S3), whereby the nozzle hole is closedwith the needle valve 13. Thus, the auxiliary injection at a lowpressure ends.

The fuel thus injected into the combustion chamber of the engine doesnot necessarily burn at once. However it becomes chemically activated,which facilitates ignition of fuel subsequently supplied to the engineby the main injection, or in. other words, decreases ignition delay. Theauxiliary injection period ΔTL is determined to supply the minimumquantity of fuel required to prevent the ignition delay, in order toimprove fuel economy. As already described based on FIG. 1, the fuelspray formed by the low-pressure auxiliary injection is distributed inan area from the vicinity of the fuel injection nozzle to a radiallyintermediate part of the cylinder. With such a fuel spray distribution,flames spread properly in the cylinder. Specifically, ignition of fuelstarts on the radially inner side of the cylinder, and flames spreadoutward from the inner area to the entire area in the cylinder.

When the auxiliary injection is performed at a low pressure as describedabove, the required accuracy of auxiliary-injection time control isalleviated than when the auxiliary injection is performed at a highpressure. This allows the fuel injection quantity in the auxiliaryinjection to be more accurately controlled to the required minimum,thereby contributing to improvement in fuel economy.

Next, in order to fill the fuel chamber 12 of the injector 9 and thedownstream section of the fuel passage 10 a with intermediate pressurefuel for intermediate-pressure injection in the initial stage of themain injection, high pressure fuel for forming the intermediate pressureis fed from the high-pressure accumulator 3 to the downstream section ofthe fuel passage 10 a.

Specifically, when the high-pressure fuel feed start timing t3determined in step S2 is reached, the selector-valve drive signal isturned on. Consequently, the selector valve 5 for injection rateswitching is opened (step S4), so that the high-pressure fuel from thehigh-pressure accumulator 3 opens the check valve 32, flows into thedownstream section of the fuel passage 10 a, and then flows into thecontrol chamber 11 and the fuel chamber 12 of the injector 9. As aresult, as shown in the lowest part of FIG. 6, the injector inletpressure increases from the injection pressure in the auxiliaryinjection (low pressure).

When the high-pressure fuel feed period ΔTm has elapsed from thefuel-pressure feed start timing t3 so that the high-pressure fuel feedend timing t4 is reached, the selector-valve drive signal is turned off,with the judgement that the intermediate pressure has been formed in thefuel chamber 12 and in the downstream section of the fuel passage 10 a,and the selector valve 5 is closed (step S4). Thus, the supply ofhigh-pressure fuel for forming the intermediate pressure ends.

Part of the intermediate pressure fuel in the fuel chamber 12 and thedownstream section of the fuel passage 10 a gradually flows into thelow-pressure accumulator 4 through the orifice 6 a. Therefore, as shownin FIG. 6, the injector inlet pressure gradually decreases a little.Although it is not clear in FIG. 6, the injector inlet pressuredecreases more steeply than before when the injector drive signal isturned on at the time point of t5 to open the on-off valve 7.

When the main injection start timing t5 is reached, the injector drivesignal is turned on to open the on-off valve 7 for injection timingcontrol (step S5). Hence, the nozzle hole of the injector 9 is caused toopen, and the intermediate pressure fuel in the fuel chamber 12 isinjected into the combustion chamber of the engine. Since the maininjection fuel is supplied in the presence of fuel supplied by theauxiliary injection and activated until that time, the main injectionfuel ignites immediately. Typically, the main injection fuel ignitesduring the shift from intermediate pressure injection to high-pressureinjection, or prior to or subsequent to the shift. As already describedbased on FIG. 2, the fuel spray formed by the intermediate-pressure maininjection reaches an area radially outwardly of the fuel spray andflames formed by the low-pressure auxiliary injection, and properlyburns consuming the air remaining in that area. According to the presentembodiment, moreover, the injection pressure in theintermediate-pressure main injection is varied depending on the engineoperating condition referring to the MP map, thereby establishing aproper combustion speed of main injection fuel in the initial combustionstage.

By conducting the intermediate pressure injection in the initial stageof main injection following the low-pressure auxiliary injection asindicated by a solid line in FIGS. 6 and 8, the fuel injection quantityin the initial stage of main injection increases as compared with thecase where the low-pressure injection is made in the initial stage ofmain injection (as indicated by a broken line in FIG. 8). Since the fuelquantity to be injected in the remaining period of main injectiondecreases by the quantity corresponding to the increase in injectionquantity in the initial stage, the period of the entire main injectionis shortened. Thus, the fuel injection ends early and fuel economy isimproved. In addition, unlike the case where high-pressure injection isstarted immediately from the initial stage of main injection, excessivefuel is prevented from being supplied before ignition, thereby reducingthe engine noise and NOx emission. Since the intermediate pressureinjection can be made between auxiliary injection and main injectiononly by opening/closing the selector valve 5 for injection rateswitching, the apparatus is not required to have a particularlycomplicated structure.

When the intermediate pressure injection period ΔTM has elapsed from themain fuel start timing t5 so that the selector-valve drive signalturning-on timing t6 is reached, the selector valve 5 for injection rateswitching is opened while the on-off valve 7 for injection timingcontrol is open (step S6). Consequently, high-pressure fuel is suppliedfrom the high-pressure accumulator 3 through the fuel passage 10 a tothe fuel chamber 12, to be injected from the injector 9 (FIGS. 6 and 8).Thus, the fuel injection (high-pressure main injection) is performed ata higher fuel injection rate than that for the intermediate pressureinjection. As already described based on FIG. 3, the fuel injected bythe high-pressure injection properly diffuses in the cylinder, involvingsurrounding air and fuel spray, especially, combustion residues such assoot, formed by the auxiliary injection and intermediate-pressure maininjection, thereby activating combustion in the cylinder andcontributing to proper combustion and reduction in black smoke.

When the main injection period ΔTMH has elapsed from the main injectionstart timing t5 so that the fuel injection end timing t7 is reached, theinjector drive signal is turned off and the on-off valve 7 for injectiontiming control is closed (step S7). Consequently, the sum of the forceby the high-pressure fuel in the control chamber 11 acting on thehydraulic piston 14 and the force by the spring acting on the needlevalve becomes larger than the force by the fuel in the fuel chamber 12acting on the needle valve 13. As a result, the nozzle hole is closedwith the needle valve 13, and the main injection ends.

Meanwhile, with the orifice 16 configured to have a relatively largeflow path cross-section, the fuel injection rate rapidly decreases atthe fuel injection end timing t7, which contributes to reduction inblack smoke and particulate matters discharged from the engine.

Then, the selector valve 5 for injection rate switching is closed at thetime point of t8 at which a predetermined period of time has elapsedfrom the fuel injection end timing t7 (step S8). Alternatively, theselector valve 5 may be closed at the time point of t7.

On and after the time point of t8 where the selector valve 5 is closed,a considerable part of high-pressure fuel in the fuel passage 10 a onthe downstream side of the check valve 32 flows through the bypass fuelpassage provided with the orifice 6 a, into the low-pressure accumulator4 where it is utilized for low-pressure fuel formation. The rest of thehigh-pressure fuel flows into the control chamber 11 or leaks out fromaround the control chamber 11 toward the fuel tank 17. Consequently, thefuel pressure in the downstream section of the fuel passage 10 a and theinjector inlet pressure decrease to the low pressure with elapse of timeuntil the auxiliary fuel injection starts in the next fuel injectioncycle, as shown in FIG. 6.

The present invention is not limited to the above embodiment and can bemodified variously.

For example, in the embodiment, the intermediate pressure injection isalways performed in the initial stage of main injection that follows thelow-pressure auxiliary injection in each fuel injection cycle, asindicated by a solid line in FIG. 8. However, it is not inevitablynecessary to always perform the combination of low-pressure auxiliaryinjection, intermediate-pressure main injection and high-pressure maininjection in each fuel injection cycle. Alternatively, it may bepossible to selectively perform, depending on the engine operatingstate, combination of low-pressure auxiliary injection,intermediate-pressure main injection and high-pressure main injection asindicated by a solid line in FIG. 8, or combination of low-pressureauxiliary injection, low-pressure main injection and high-pressure maininjection as indicated by a broken line in FIG. 8.

What is claimed is:
 1. An accumulator fuel-injection apparatus,comprising: a first accumulator for storing high-pressure fuelpressurized by a pump; a fuel injection nozzle, connected with saidfirst accumulator through a fuel passage, for injecting fuel into acombustion chamber of an engine; a control valve for controllingdischarge of the high-pressure fuel in said first accumulator toward adownstream-side of the fuel passage; a second accumulator, connectedwith the fuel passage on a downstream-side of the control valve througha branch passage, for storing low-pressure fuel having a pressure lowerthan the pressure of the high-pressure fuel in said first accumulator;and fuel control means for opening said control valve for a short timebetween auxiliary short injection and main injection and for openingsaid control valve in the course of the main injection, while theauxiliary short injection and the main injection are carried out, inthis order with an interval between through said fuel injection nozzle.2. The accumulator fuel-injection apparatus according to claim 1,wherein said fuel control means establishes an intermediate pressure inthe fuel passage on the downstream-side of said control valve in aninitial stage of the main injection, said intermediate pressure beinghigher than a pressure of the low-pressure fuel in said secondaccumulator and lower than a pressure of the high-pressure fuel in saidfirst accumulator.
 3. The accumulator fuel-injection apparatus accordingto claim 2, wherein the intermediate pressure formed in the fuel passageon the downstream-side of said control valve in the initial stage of themain injection has a value closer to the pressure of the low-pressurefuel in said second accumulator in a lower-speed, lower-load engineoperating region, and closer to the pressure of the high-pressure fuelin said first accumulator in a higher-speed, higher-load engineoperating region.